Sheet membranes are employed in a diversity of uses which involve placing the sheets in a stacked or interleaved or layered arrangement or configuration wherein the space between layers, whether between two membranes or between a membrane and some other material, is established and maintained by a sheet separator. In practice there are a wide number of materials employed as separators, each of which may be available in a variety of forms.
By way of example, membrane separators are made of polypropylene, polyamides, polyesters, particularly polyethylene terephthalate, and the like. Such separators may be formed into a suitable sheet form for use in the form of woven or non-woven fabric or webs, perforated film, macroporous membranes, or even as sintered or partially fused particulate forms.
In such applications, the layered construction may be in flat, planar form, wound into a spiral configuration, or other arrangements involving plural layers where one or more layers is required to be a separator for other adjacent layers.
In many applications of such configurations, there is a need for sterilization of the final assembly before use. Among such applications are sterile filtration, particularly those involving food treatment or pharmaceutical products, dialysis, particularly hemodialysis and other biological and medical techniques, and in the use of membranes in bioreactors to confine living cells or organisms in the production of biological products.
It is usual to subject such membrane assemblies to autoclaving to achieve sterile conditions. By the usual conditions of high temperature and pressure, typically about 120 to about 130 deg C., at about 1.0-2.0 atmospheres of steam, it is generally rather rapid and convenient to achieve effective sterilization, although it may be necessary to limit the temperature of the steam to avoid damage to temperature sensitive materials. This can generally be offset by increased retention time at a lower temperature.
Other techniques for sterilization, i.e., cold chemical sterilizing and the like, are generally not effective, often taking many hours, and often cannot be employed at all without damage to the materials used in the stack assembly or difficulty in removing the sterilizing agent after the procedure.
It is well known to the art that most of the common materials of interest as separators in such assemblies are susceptible to a material degree of shrinkage upon autoclaving. It is a general problem which requires that the assembly be formed in such a fashion that the shrinkage is allowed for, so that the configuration is appropriate after the autoclaving operation. In some sheet membrane - separator assemblies this can be achieved with little difficulty.
There are a number of occasions where the desired assemblies require a construction that does not admit of leaving allowances for shrinkage. This most often arises in stack assemblies involving the use of a plurality of membrane layers, where each layer is separated from its neighbors by a separator sheet layer, and where the assembly requires sealing at the edges. This type of structure, whether in flat stack configurations or in spiral wound assemblies, is of considerable and increasing interest for bioreactor applications, biological dialysis and filtration applications, and the like.
In such structures, the required allowances for shrinkage of the separator cannot be made without compromising the structure in unacceptable ways.
On the other hand, if no allowance is made for the shrinkage, the structure will be materially distorted and damaged by the effects of shrinkage when the assembly is autoclaved.
Since neither of these possibilities can be tolerated, it is usual to select separator materials which are not susceptible to such distortions when subjected to autoclaving. It has often been attempted to employ pre-shrunk materials, some of which are available, but even these types of materials have been observed to retain a material susceptibility to shrinkage. The usual preshrinking procedure is to subject the screen material to high temperature "annealing" under tension. Because these techniques are not generally sufficient, it is the current practice in the manufacture of membrane stacks to employ expensive and exotic materials to avoid the shrinkage problems. For example, such materials as fluorinated hydrocarbon polymer porous sheet materials are frequently employed in such applications.
These materials are expensive, often to the deree that the cost of the separator components may exceed the total costs of the other components of the assembly by a wide margin.